foetation,--that is, to
the influence of two fathers. All the characters above enumerated,
which are transmitted in a perfect state to some of the offspring and
not to others,--such as distinct colours, nakedness of skin, smoothness
of leaves, absence of horns or tail, additional toes, pelorism, dwarfed
structure, &c.,--have all been known to appear suddenly in individual
animals and plants. From this fact, and from the several slight,
aggregated differences which distinguish domestic races and species
from {95} each other, not being liable to this peculiar form of
transmission, we may conclude that it is in some way connected with the
sudden appearance of the characters in question.

_On the Modification of old Races and the Formation of new Races by
Crossing._--We have hitherto chiefly considered the effects of crossing in
giving uniformity of character; we must now look to an opposite result.
There can be no doubt that crossing, with the aid of rigorous selection
during several generations, has been a potent means in modifying old races,
and in forming new ones. Lord Orford crossed his famous stud of greyhounds
once with the bulldog, which breed was chosen from being deficient in
scenting powers, and from having what was wanted, courage and perseverance.
In the course of six or seven generations all traces of the external form
of the bulldog were eliminated, but courage and perseverance remained.
Certain pointers have been crossed, as I hear from the Rev. W. D. Fox, with
the foxhound, to give them dash and speed. Certain strains of Dorking fowls
have had a slight infusion of Game blood; and I have known a great fancier
who on a single occasion crossed his turbit-pigeons with barbs, for the
sake of gaining greater breadth of beak.

In the foregoing cases breeds have been crossed once, for the sake of
modifying some particular character; but with most of the improved races of
the pig, which now breed true, there have been repeated crosses,--for
instance, the improved Essex owes its excellence to repeated crosses with
the Neapolitan, together probably with some infusion of Chinese blood.[200]
So with our British sheep: almost all the races, except the Southdown, have
been largely crossed; "this, in fact, has been the history of our principal
breeds."[201] To give an example, the "Oxfordshire Downs" now rank as an
established breed.[202] They were produced about the year 1830 by cro